Anshy Oonnittan Plamthottathil
APPLICATION OF ELECTROKINETIC FENTON PROCESS FOR THE REMEDIATION OF SOIL CONTAMINATED WITH HCB
Acta Universitatis Lappeenrantaensis 468
Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in the Auditorium of Mikkeli University Consortium on the 4 th of May, 2012, at 12:00.
Supervisor Prof. Mika Sillanpää
Lappeenranta University of Technology Finland
Reviewers Dr Claudio Cameselle Associate Professor
Department of Chemical Engineering University of Vigo
Spain
Prof. Dr. –Ing. Wolfgang Calmano Hamburg University of Technology
Institute of Environmental Technology and Energy Economics Germany
Opponent Dr Claudio Cameselle Associate Professor
Department of Chemical Engineering University of Vigo
Spain
ISBN 978-952-265-211-9 ISBN 978-952-265-212-6 (PDF)
ISSN 1456-4491
Lappeenrannan teknillinen yliopisto Digipaino 2012
ABSTRACT
Anshy Oonnittan
Application of electrokinetic Fenton process for the remediation of soil contaminated with HCB Lappeenranta, 2012
67 p.
Acta Universitatis Lappeenrantaensis 468 Diss.Lappeenranta University of Technology ISBN 978-952-265-211-9, ISBN 978-952-265-212-6 (PDF) ISSN 1456-4491
Electrokinetic remediation coupled with Fenton oxidation, widely called as Electrokinetic Fenton process is a potential soil remediation technique used for low permeable soil. The applicability of the process has been proved with soil contaminated with a wide range of organic compounds from phenol to the most recalcitrant ones such as PAHs and POPs.
This thesis summarizes the major findings observed during an Electrokinetic Fenton Process study conducted for the remediation of low permeable soil contaminated with HCB, a typical hydrophobic organic contaminant. Model low permeable soil, kaolin, was artificially contaminated with HCB and subjected to Electrokinetic Fenton treatments in a series of laboratory scale batch experiments. The use of cyclodextrins as an enhancement agent to mobilize the sorbed contaminant through the system was investigated. Major process hindrances such as the oxidant availability and treatment duration were also addressed. The HCB
degradation along with other parameters like soil pH, redox and cumulative catholyte flow were analyzed and monitored.
The results of the experiments strengthen the existing knowledge on electrokinetic Fenton process as a promising technology for the treatment of soil contaminated with hydrophobic organic compounds. It has been demonstrated that HCB sorbed to kaolin can be degraded by the use of high concentrations of hydrogen peroxide during such processes. The overall system performances were observed to be influenced by the point and mode of oxidant delivery.
Furthermore, the study contributes to new knowledge in shortening the treatment duration by adopting an electrode polarity reversal during the process.
Keywords: electrokinetic, electrokinetic Fenton, soil remediation, hexachlorobenzene, cyclodextrin, polarity reversal, hydrogen peroxide.
UDC 502/504:504.5:541.17:628.516
Acknowledgements
I extend my heartfelt gratitude to my supervisor Prof. Mika Sillanpää, a veteran and award winner in the field of Environmental Technology, for his support, supervision and guidance throughout my whole PhD tenure under him. I thank you for giving me an opportunity to work under you and fulfill a lifetime ambition.
I would like to thank my mentor and co-supervisor, Dr Reena A. Shrestha, for giving me courage and guidance, especially during the initial phases of my studies. I express my respect and profound gratitude to my second co-supervisor Dr Pirjo Isosaari for her guidance, suggestions and comments which have helped me to draw major conclusions in many instances. I am indebted to you for giving me a chance to work under you at TKK.
All my colleagues in the laboratory are remembered for their acceptance and cooperation. Special thanks to Heikki Särkkä, who was always there with a helping hand and a smiling face. No words to express my feelings to Thuy Duong (Daisy), the honest and the most self-less being i have ever met.
Special thanks for the financial assistance by the funding bodies arranged by Prof. Mika Sillanpää and the personal grant from Maa- ja vesitekniikan tuki ry (MVTT).
I am very much thankful to my parents, Mr. P. P. Oonnittan and Mrs. Mariyamma Oonnittan as well as my brother, Mr. Anish Oonnittan who have given me the mental support for my stay in Finland and my studies here. My father‘s highly ambitious expectations for his daughter have encouraged and helped me to achieve this milestone in my career.
I thank my loving husband, Mr. Hans James, for his continual support without which i would have never achieved my goal. My sweet little angel, Sara, is thanked for being my stress reliever and energy booster. You have made my life brighter and i love you so much!
LIST OF PUBLICATIONS
The publications are listed in roman numerals in the text as follows:
I. Anshy Oonnittan, Reena Shrestha, Mika Sillanpää, Remediation of hexachlorobenzene in soil by enhanced electrokinetic Fenton process. Journal of Environmental Science and Health Part A 43 (2008) 894-900.
II. Anshy Oonnittan, Reena Shrestha, Mika Sillanpää. Removal of hexachlorobenzene from soil by electrokinetically enhanced chemical oxidation. Journal of Hazardous Materials 162 (2009) 989-993.
III. Anshy Oonnittan, Reena Shrestha, Mika Sillanpää, Effect of cyclodextrin on the remediation of hexachlorobenzene in soil by electrokinetic Fenton process, Separation and Purification Technology 64 (2009) 314–320.
IV. Anshy Oonnittan, Pirjo Isosaari, Mika Sillanpää, Oxidant availability and its effect on HCB removal during electrokinetic Fenton process, Separation and Purification Technology 76 (2010) 146-150.
V. Anshy Oonnittan, Pirjo Isosaari, Mika Sillanpää, The effect of polarity reversal on HCB removal during electrokinetic Fenton process, submitted.
Author’s contribution in the publications
Planned, designed, carried out all the experiments, analyzed the data, interpreted the results and prepared the first draft of all the manuscripts.
TABLE OF CONTENTS
1 Introduction ...13
2 Study Background ………....15
2.1Electrokinetic soil remediation ... 15
2.1.1 Principles of the technology……….. .15
2.1.2 Treatment of organic pollutants………...18
2.2 Electrokinetic Fenton Process………...23
2.3 Hexachlorobenzene………...29
2.3.1 Environmental properties………... 29
2.3.2 Sources in the environment……….. 31
2.3.3 Health effects………. ..32
3 Objectives of the study... 33
4 Materials and Method... 34
4.1 Design of apparatus...34
4.2 Soil characterisation...35
4.3 Test specimen... 36
4.4 Experimental Program... 36
4.5 Chemical analyses………...40
5 Results & discussions………..42
5.1 Sorption studies……….42
5.2 Feasibility Tests – Electrokinetic and Electrokinetic Fenton Treatments……….42
5.3 Electrode positioning / Type 1 and Type 2 apparatus………...42
5.4 Electrokinetic Fenton Treatment with and without β-cyclodextrin………..45
5.5 Oxidant availability – Electrokinetic Fenton Treatment with different modes and points of oxidant addition………...47
5.6 Electrokinetic Fenton treatment with polarity reversal………...48
5.7 General ………...49
5.7.1pH………49
5.7.2 Redox potential……….50
5.7.3 Electroosmotic flow……….50
5.8 Significance of the obtained results………..51
6 Conclusion………... 53 References
Appendix
Symbols
q Volume flow rate m3/s
Ke electroosmotic permeability of the soil m2/Vs
E electric field strength V/m
A cross-sectional area of the soil sample m2
OH* hydroxyl radical
HO2* perhydroxyl radical.
R* organic radical
Q mass of HCB sorbed per unit mass of kaolin mg/kg C concentration of HCB in solution at equilibrium mg/l
Abbreviations
POPs persistent organic pollutants PAHs polycyclic aromatic hydrocarbons
HCB hexachlorobenzene
TDI tolerable daily intake
USEPA US Environmental Protection Agency
ATSDR Agency for Toxic Substances and Disease Registry
DW deionized water
OCPs organochlorine pesticides HPCD hydroxypropyl beta cyclodextrin EDTA ethylenediamine tetraacetic acid DTPA diethylenetriamine pentaacetic acid SDS sodium dodecyl sulfate
APG alkyl polyglucosides
1 INTRODUCTION
Soil constitutes one of the major parts of the ecosystem. Activities that lead to soil contamination may also disrupt the soil quality. Industrial development and agricultural activities have been pointed out as the major causes for soil contamination so far. The increased awareness about the threats caused by organic contaminants especially POPs have raised concerns and have led the governments and authorities to put stringent regulations on their manufacture and use. Once, POPs get into the soil, they are difficult to remove due to their hydrophobic nature.
Numerous techniques based on physical, chemical and biological methods have been in practice for the remediation purposes. However, they have often proved unsuccessful when the site consisted of heterogeneities or low permeable soil [1].
Electrokinetic technology has been developed and accepted as an effective remediation technique for treating low permeable soils [2]. Electrokinetic remediation has been applied for the removal of a wide range of contaminants, including heavy metals, organics and radionuclides. The use of electrokinetics for the removal of organic contaminants can be beneficial when aimed for the complete destruction/degradation of the contaminants right in the soil itself. This can be accomplished by suitably combining chemical oxidation processes like Fenton oxidation with electrokinetic treatment of soil.
Integrated technologies based on electrokinetics like electrokinetic Fenton process have shown their potential for addressing such technologically challenging situations. Despite the need for such integrated processes for solving the pollution issues, this area remains largely under - researched. Therefore, it is of great importance that studies pertaining to the applicability of
electrokinetic Fenton processes for the degradation of sorbed organic contaminants be carried out.
2 STUDY BACKGROUND 2.1. Electrokinetic soil remediation
2.1.1. Principles of the technology
Electrokinetics has emerged as one of the most versatile technologies for soil remediation over the past three decades because of its suitability to treat both inorganic and organic pollutants as well as radionuclides, saturated and unsaturated soil matrices, low permeable soil and heterogeneous soil layers.
Electrokinetic treatment has shown its potential also for the simultaneous removal of inorganic and organic species from soil [3, 4]. Electrokinetics is a process that can be used to decontaminate soil by driving the inorganic and organic contaminant species through the soil matrix. The driving force for this species transfer is the applied electric field. Finally, the contaminants are removed by electroplating at the electrode, precipitation or co-precipitation at the electrode, adsorption onto the electrode or complexing with ion exchange resins (heavy metal species) and pumping water near the electrode (organic contaminants) [5].
Several large scale applications of electrokinetics were reported even from late 1970s and early 1980s [6]. These studies were based on the fundamental aspects of the technology without the comprehension of complicated electrochemical phenomena that actually govern the process.
However, the results of these applications proved the potential of electrokinetics for the removal of a wide range of inorganic pollutants, soluble organic pollutants and radionuclides [7, 8, 9, 10, 11, 12]. Later on, the focus was directed to the enhancement of the processes for better removal rates, including pH conditioning and surfactant additions [13, 14, 15]. At present, several pilot
scale trials are being carried out for the removal of organic compounds, heavy metals and radio nuclides from contaminated soil. [16, 17, 18]
The recent developments and the history of electrokinetics, how it emerged and evolved as a remediation technique, have been explained by Yeung [19].
The principles of electrokinetic technology have elaborately been explained and thoroughly understood [2, 5, 20, 21]. There are several electrochemical phenomena taking place upon the application of electric field on a soil mass. However, the major transport mechanisms that are relevant from a remediation standpoint are the following:
1. Electromigration 2. Electroosmosis 3. Electrophoresis
Electromigration refers to the movement of charged species present in the soil mass under the influence of an externally applied electric field. The charged species move towards the electrodes of opposite polarity. The ionic mobilities of heavy metals at infinite dilution are in the range of 10 -4 cm2/Vs. However, taking into account the effective ionic mobility due to the tortuosity in a porous medium like soil, the rate of heavy metal transport in clayey soil is about a few centimeters per day under a unit electric gradient [22].
Electroosmosis is the movement of the pore fluid under an electric field which results from the interaction between the bulk liquid and the diffuse double layer existing at the soil particle /fluid interface. The direction of the electroosmosis depends on the surface charge of the soil particles.
Since, a negative surface exists in most soil particles, especially on clayey soil, excess positive charges are distributed adjacent to the soil surface which continuously drags the bulk fluid
towards the cathode. Therefore, the direction of electroosmosis is always towards the electrode of negative polarity unless, the surface charge of the soil particles are changed. The rate of electroosmotic flow in a porous medium is defined by the Helmholtz-Smoluchowski equation which is explained in latter section.
Electrophoresis is the transport of charged particles (like clay particles or microorganisms) or colloids under an applied electric field. However, electrophoresis has no major role in a compact solid phase, where there is minimal movement of the particles.
A recent review by Mahmoud et al. [23] presents a detailed account of these electrokinetic transportation phenomena.
Highly soluble ionized inorganic species that are present in moist soil environments are transported by electromigration and also by electroosmosis depending upon the species concentration [5]. However, electroosmosis plays the dominant role in the transport of soluble organic species present in the soil.
Besides these transportation processes, the application of voltage also leads to other electrode reactions and the corresponding geochemical reactions in the treated material. The electrode reactions involve the electrolysis of water:
2H2O – 4e- = O2 + 4H+ (anode) (1)
2H2O+ 2e- = H2 + 2OH- (cathode) (2)
The protons and hydroxyl ions generated at the anode and cathode, respectively, are transported through the soil. The mobility of protons under an applied electric field is about two times the mobility of hydroxyl ion [24]. Also, the advance of the basic front developed at the cathode is
retarded by the counteracting electroosmotic flow. Therefore, the soil pH that develops during an electrokinetic process depends on the extent of the movement of protons and hydroxyl radicals as well as the geochemical characteristics of the soil such as its buffering capacity [25]. These changes in the soil pH lead to other geochemical reactions in the soil which include sorption- desorption reactions, complexation reactions, precipitation –dissolution reactions and oxidation- reduction reactions.
These geochemical reactions significantly affect the electrokinetic process and can enhance or retard the process [22].
Several techniques have been proposed to alter the normal geochemical reactions that arise from the electrode reactions to enhance the electrokinetic removal of species from the soil. They are controlling the soil pH by suitable acid, base, and buffer additions into the anolyte or catholyte, by using specially designed ion selective membranes, or by using enhancement agents which aid in the complexation and removal of species present etc [15, 22 ].
2.1.2. Treatment of organic pollutants
A major part of the early research on electrokinetics was devoted to the investigation of heavy metal removal. Removal of organic pollutants by electrokinetic process gained interest and attention only when the role of electroosmosis in transporting the soluble organics was understood.
Among different theories proposed for the electroosmotic flow, including the Helmholtz- Smoluchowski theory, Schmid theory, the Spiegler friction model, and ion hydration theory, the
Helmholtz-Smoluchowski theory is the most common theoretical description of electroosmosis [22]. The rate of electroosmotic flow is controlled by the coefficient of electroosmotic permeability of soil [24]. Ke, as stated by Helmholtz- Smoluchowski equation depends primarily on the porosity and zeta potential of soil and can be assumed as constant during an electrokinetic process as long as there is no change in the concentration of ions or pH of the pore fluid.
For practical purposes, electroosmotic flow rate is expressed by an equation analogous to Darcy’s law of hydraulic flow as:
q = ke E A
The very first reports on the application of electroosmosis for the decontamination of soils contaminated with organic compounds were presented by the research team led by R. F.
Probstein [26, 27]. They demonstrated the feasibility of using electroosmosis for the removal of organic compounds like phenol and acetic acid from saturated clay both theoretically and experimentally.
However, the process had limited application when dealing with insoluble organics sorbed to the soil [2]. This is because electroosmosis is expected to transport only the hydrophilic organic contaminants. The focus then shifted to the removal of hydrophobic organic contaminants, and the use of solubilizing agents to desorb and mobilize insoluble organic contaminants gained attention. Subsequent research on the electrokinetic treatments made use of these enhancement agents like surfactants, cosolvents or cyclodextrins to treat sorbed contaminants [28, 29, 30].
Surfactants are compounds with a polar hydrophilic head and a non polar hydrophobic tail. Based on the hydrophilic head group surfactants are commonly classified as anionic, cationic and non ionic. Among these, nonionic surfactants are generally considered ideal for soil remediation as
they have higher solubilization capacities and are relatively non-toxic [31]. Surfactants increase the aqueous solubility of organic compounds by lowering the interfacial tension and by micellar solubilization [32]. Cosolvents are organic solvents that are capable of changing the aqueous phase characteristics of organic compounds such as its solubility, sorption kinetics and transport velocity [31]. Cyclodextrins, formed by the degradation of starch by bacteria, are linear chain glucose molecules with their ends joined to form a cyclic structure. They form inclusion complexes with hydrophobic organic compounds by partitioning them to the center of their ring, thus significantly enhancing the aqueous solubility of organic compounds [33]. Recently there have been more interest towards cyclodextrin based enhancements. This is because they are non toxic and biodegradable when compared to toxic surfactant micelles and cosolvents at higher concentrations [34]. Moreover, cyclodextrins have been proved effective for the mobilization of PAHs and other HOCs in contaminated soil [3, 35].
Electroosmotic flushing of solubilizing agents through the soil can be used as such to remove soil-bound organic contaminants. A summary of some of the studies undertaken during the last decade in electrokinetic remediation of organic compounds using these kinds of enhancement agents is presented in Table 1.
Table 1. Summary of electrokinetic soil remediation studies using enhancement agents.
Contaminant Soil
Type/Source Enhancement
agent used Relevant Results Reference
Gas oil Natural soil Rhamnolipid A maximum of 86.7% removal with the highest dose used. [36]
HCB, Heavy
metals Aged
sediment HPCD Nearly 40 % HCB removal
efficiency obtained with 2.6 pore volumes.
[37]
Diesel oil Petrol station
soil EDTA, n-propanol,
Tergitols EDTA alone enhanced the removal of aliphatic and aromatic
compounds in the soil. A combination of n-propanol and EDTA enhanced the hydrocarbon removal efficiency.
[38]
Phenanthrene collected from specific waste site
Triton X - 100 and
Rhamnolipid Rhamnolipid found to be more
efficient than Triton X- 100 [39]
Phenanthrene,
Nickel Kaolin Igepal CA-720,
Tween 80 Complete removal observed using 5
% Igepal CA-720 [40]
Phenanthrene,
Lead, Zinc MGP soil Igepal CA-720, Tween 80, n- butylamine, tetrahydro furan, EDTA.
Effective removal of phenanthrene observed with different
concentrations of Igepal CA-720 and Tween 80.
[41]
Phenanthrene,
Nickel Kaolin n-butylamine Significant solubilisation of phenanthrene with increasing concentrations of the cosolvent.
[42]
Phenanthrene Kaolin APG, Brij 30, SDS APG found to be the best among three in terms of removal efficiency and electroosmotic flow.
[43]
DDT Sandy loam Tween 80, SDBS Though both surfactants showed similar solubilization potentials for DDT, electrokinetics transport with SDBS yielded better results.
[44]
PAH mixture,
Heavy metals MGP soil Tween 80, Igepal CA-720, n- butylamine, HPCD
Though highest electroosmotic flow was observed with cosolvents, Igepal CA 720 resulted in the highest removal efficiency.
[45]
HCB Kaolin β-cyclodextrin,
Tween 80 Better removal with β-cyclodextrin
than with Tween 80. [35]
2, 4
dinitrotoluene Kaolin,
Glacial till HPCD Higher degradation with HPCD in
kaolin. [46]
Phenanthrene,
Nickel Kaolin HPCD Phenanthrene removal was high
when using 1 % HPCD compared to 10 % HPCD. However, the overall removal was not high due to the low concentration of HPCD.
[3]
Table contd.
The application of these electrokinetic processes made a major leap forward when organic contaminants were successfully removed and degraded/destroyed from the soil matrix. This was made possible by coupling electrokinetics with other remediation techniques like biodegradation and chemical oxidation. Electrokinetics coupled with bioremediation is an effective technique if the treatment duration is not a primary concern for the remediation project. However, it fails when the target contaminant is toxic and present at high concentrations. Integrated technologies Contaminant Soil
Type/Source Enhancement
agent used Relevant Results Reference
Phenanthrene,
Lead, Zinc MGP soil Igepal CA-720, n- butylamine, tetrahydrofuran, EDTA, DTPA
Surfactants, Igepal CA-720 and Tween 80, effective in removing phenanthrene.
[47]
Phenanthrene Kaolinite APG, Calfax 16L-35 Though both showed similar solubilisation potentials, electrokinetics movement of phenanthrene with increasing concentrations of APG resulted in higher removal.
[48]
Ethybenzene Clayey
natural soil Mixture of SDS,
PANNOX 110 A removal efficiency of 63-98 %
observed. [28]
Phenanthrene,
Nickel Kaolin Surfactants (Igepal CA-720, Tween 80), Cosolvents (n- butylamine, tetrahydrofuran), cyclodextrins (HPCD, β- cyclodextrin hydrate), chelating agents (EDTA, DTPA)
Surfactants and cosolvents were found to be effective for the removal of phenanthrene.
[49]
Phenanthrene Kaolin,
Glacial till Igepal CA-720, Triton X-100, Tween 80, ethanol, ethanol-Igepal mixture.
Highest desorption and solubilization observed with the surfactant solutions.
[50]
Phenanthrene Kaolin,
Glacial till. Tween 80, ethanol Contaminant desorption possible by surfactant or cosolvent solution. [51]
Phenanthrene Glacial till n-butylamine, tetrahydrofuran, acetone
43 % phenanthrene removal after
127 days or 9 pore volumes. [52]
based on advanced oxidation processes have proved to be very successful in treating organic contaminants, especially toxic ones. A comprehensive study on integrated electrokinetic chemical oxidation processes using different oxidants such as sodium persulfate and Fenton’s reagent was performed by Isosaari et al [53]. They observed that during the electrokinetic treatment with persulfate oxidation, 35 % of the total PAH mixtures were removed from the soil near the anode section in 8 weeks. However, electro Fenton test did not result in a better performance than electrokinetics alone. Electrokinetic- permanganate oxidation for the removal of phenol was studied by Thepsithar & Roberts [54]. According to their results, 90 % of phenol was removed from the soil during a 5 days treatment with a voltage gradient of 1 V/cm. Another study by Pham et al [55] used Ultrasound as an enhancement during the electrokinetic removal of phenanthrene, fluoranthene and HCB. Their results were encouraging and proved the effectiveness of ultrasonically enhanced electrokinetic remediation of soil. However, the results indicate that HCB was the most difficult to treat, probably because of its extremely stable nature.
Yang & Yeh [56] recently reported the feasibility of electrokinetically enhanced persulfate oxidation for the destruction of TCE in a spiked sandy clay soil. Also, the role of nanoscale Fe3O4 for activating persulfate was investigated. By doing so, they have achieved the target concentration well below the regulatory threshold values in the soil as well as in the electrode chambers. However, limited studies were done so far for the remediation of stable and recalcitrant organic compounds like OCPs. Moreover, degradation of stable OCPs necessitates the application of aggressive and rigorous chemical oxidation processes such as the Fenton process.
2.2 Electrokinetic Fenton Process
Electrokinetic Fenton process is an integrated technology incorporating chemical oxidation by Fenton’s process with the electrokinetic treatment of soil. The applicability of iron-catalyzed H2O2 as an oxidizing agent was first reported by H. J. H. Fenton [57]. Several works have been documented on the use of Fenton’s process for the oxidation of organic compounds including recalcitrant contaminants [58, 59, 60, 61, 62, 63]. The use of Fenton’s process is favored also due to the fact that the final reaction products are environmentally benign.
The primary reactions in the Fenton’s process are:
H2O2 + Fe2+ → OH* + OH- + Fe3+ (3)
H2O2 + Fe3+ → HO2* + H+ + Fe2+ (4)
OH* + Fe2+ → OH- + Fe3+ (5)
HO2*+ Fe3+ → O2 + H+ + Fe2+ (6)
H2O2 + OH* → H2O + HO2* (7)
In the presence of organic substrate the reactions include:
RH + OH* → R* + H2O + HO2* (8)
R* + Fe3+ → Fe2+ + degradation products (9)
Thus Fenton’s oxidation is an effective mechanism for the decomposition of toxic organic compounds [64, 65, 66, 67, 68]. However, when applied alone, Fenton’s process fails to treat low permeable soil. This is because effective contact between the oxidant and the contaminant is a primary requirement for a successful treatment which is not possible in matrices of low
permeability. This drawback can be overcome by integrating electrokinetics with Fenton’s process. In electrokinetic Fenton process, hydrogen peroxide passes through low permeable soil from anode to cathode by electroosmosis and decomposes the contaminants in the soil in the presence of iron present in the soil. The attractiveness of this coupled technology is that it addresses one of the major shortcomings of eletrokinetic remediation by removing as well as destroying/degrading the contaminants, thus avoiding a further treatment or disposal of the waste stream.
The application of electrokinetic Fenton technology for the remediation of contaminated soil was first reported by Yang and Long [69]. In their study a saturated sandy loam containing phenol as pollutant was treated by electrokinetic Fenton by incorporating a permeable reactive bed containing scrap iron powder into the soil bed and H2O2 was flushed from the anode reservoir.
This was followed by a more elaborate study on the performance of electrokinetic Fenton technology for the oxidation of TCE in two types of soil [70]. The results were interpreted for two different types of electrodes and the form and type of iron catalyst used. These studies formed the basis of several successive research based on electrokinetic Fenton process for soil remediation by different research groups. A summary of these studies is presented in Table 2.
The hydroxyl radicals generated in Fenton’s reaction are generated in aqueous solutions and are capable of oxidizing the contaminants in aqueous solution [71]. Therefore they are unable to attack the contaminants sorbed to the soil. However, it has been documented that oxidation of sorbed contaminants in the subsurface can actually be promoted by using a high concentration of H2O2 (> 2%) [58, 67].
This is because the use of high concentrations of H2O2 favors the generation of highly reactive species other than hydroxyl radical like hydroperoxide radicals (HO*2 ), superoxide anions (O2*-) and hydroperoxide anions (HO2*-) which are capable of degrading even the most recalcitrant compounds in the sorbed form [72, 73 ].
H2O2 + OH* → H2O + HO*2 (10)
HO*2 → O*2 + H+ (11)
HO*2 + Fe2+ → HO2*-+ Fe3+ (12)
Studies by Ferrarese et al. [72] and Rivas [73] suggest that the generation of non/hydroxyl radicals such as hydroperoxide radicals (HO*2), superoxide anions (O2*-) and hydroperoxide anions (HO2*-) are responsible for the aggressive chemical reactions which ultimately lead to the oxidation of sorbed contaminants.
Another explanation is that a high concentration H2O2 first desorbs the contaminants from the soil surface and then oxidizes them. Kawahara et al. [74] proposed that high concentration of H2O2 has the ability to extract PAHs from clays. The mechanism they have explained is that the electron exchange by structural iron in clay mineral results in the swelling of clay layers and this swelling increases the space between the layers and release the sorbed contaminants. All these studies have established the effectiveness of high concentration of H2O2 in the remediation of soil contaminated with sorbed contaminants. Also, these studies reveal that sorbed hydrophobic contaminants can be treated without any enhancing agents if high concentrations of H2O2 are used.
As seen from Table 2, most of the studies on electrokinetic Fenton treatments were focused on the remediation of soluble organics and relatively insoluble PAH mixtures. Studies on remediation of soil polluted with OCPs like HCB by electrokinetic oxidation treatment are limited.
The success and performance of such in situ processes rely on certain key factors including oxidant selection, oxidant loading and oxidant delivery. Numerous studies have been conducted on the oxidant loading or in other terms the dosing of Fenton’s reagent for the oxidation of a variety of soil contaminants like phenanthrene, pyrene, chrysene etc [73]. On the other hand, little attention has been given on the studies on oxidant delivery, especially during electrokinetic treatment of soil. Oxidant delivery is important since it determines the extent to which the contaminated soil comes into contact with the oxidant. Therefore, the oxidant should be delivered to the soil in such a way so as to facilitate effective soil-oxidant interaction. It was also pointed out by Isosaari et al [53] that higher oxidation rates were observed near the oxidation injection points in their experiments. This emphasizes the importance of oxidant delivery during these processes.
The environmental impacts of Fenton treatment as discussed by Yap et al [75] show how important it is to restore the soil properties in the post treated soil in order to sustain the microbial activities and soil vegetation. This is because, electrokinetically treated soils in most cases result in an acidic soil which may lead to metal dissolution and also unavailability of plant nutrients at low pH [76]. Non-uniform electrokinetics induced by reversing the electrode polarity has been studied for maintaining the soil pH and also for improving the mobility of organic pollutants [77, 78]. Therefore, such a polarity reversal can be adopted to enhance the electrokinetic Fenton
process by propagating the oxidant through the soil matrix in a better way and also result in a more uniform pH throughout the soil section.
Table 2. Summary of electrokinetic Fenton based research in soil remediation.
Contaminant Soil Type Applied Voltage (V/cm)
% of H2O2
used
Duration
(days) Relevant results Reference
Phenol Sandy
loam 1 0.3 10 A maximum of 99.7 %
destruction of the contaminant was obtained .
[69]
TCE Loamy
sand and Sandy loam
1 <4000
mg/l 10 A maximum destruction of 59.4 % achieved. [70]
Phenanthrene Sandy soil 1.2 5 21 A maximum removal of 81.6
% achieved. [80]
Phenanthrene kaolinite 1.5 7 10 A maximum of 74 %
removal with acid ingestion. [81]
Phenanthrene Hadong clay and EPK kaolin
1.5 7 22 A maximum of about 50 %
removal attained even at the cathode region when H2SO4
was also used in the analyte.
[82]
Phenanthrene Hadong
clay 1.5 7 10 -22 Better treatment efficiency
observed with the use of phosphate stabilizer and an anionic surfactant.
[83]
PAH mixture Creasote contaminat ed real clay
0.48 and a 10 % AC component
3 60 No significant benefit
observed from Electrokinetic Fenton treatment
[53]
Phenanthrene,
Nickel Kaolin 1 5, 10, 20,
30 30 A maximum of 56 %
oxidation of phenanthrene with 30 % H2O2 .
[83]
PAHs Kaolin,
MGP soil 2 5 -10 5 – 8 for
kaolin, 25 for MGP soil
A maximum of 90.5 % phenanthrene oxidation in kaolin when ethanol was pre-flushed and then treated with 5 % H2O2 from anode and 1.4mM Fe-EDTA from the cathode.
[84]
Diesel
Phenanthrene
Subsurface layer soil
Kaolinite 4
3
0, 4, 8
10
60
14
A maximum of 42 % with the highest H2O2
concentration used.
An overall extraction and destruction efficiency of 99
% was achieved when both the electrode reservoirs were filled with H2O2.
[85]
[86]
2.3. Hexachlorobenzene 2.3.1. Environmental properties
POPs are organic compounds categorized as a special group based on their salient properties.
These salient features which make an organic compound a POP are its:
Persistence - POPs are extremely resistant to physical, chemical and biological degradation. They have long half lives in soil, water and air.
Bioaccumulation – They accumulate in the organism to a level which can be harmful to the human health and environment.
Ability for long range transport – It can be transported by the environmental media to far sites where they have never been used or produced, such as in the Arctic regions.
Toxicity – POPs are extremely toxic and pose a threat both to the human health and the environment.
Some of their physical and chemical properties which determine their fate in the environment are their low water solubility, high lipid solubility, high molecular mass and low volatility.
The organic pollutants that constitute the POPs can be broadly classified into two categories, intentionally manufactured POPs and unintentionally produced byproducts [87].
HCB is a typical POP which can be regarded as a representative compound for studies. It comes under both the classifications of POPs, since it was previously manufactured on a large scale and still produced as a byproduct during the manufacture of other chlorinated solvents. It is also one of the 12 priority pollutants listed by the Stockholm convention on POPs [88].
Hexachlorobenzene is a white crystalline solid that is practically insoluble in water. Its chemical structure is as shown in Figure 1. Physical and chemical properties of Hexachlorobenzene are shown in Table: 3
Fig: 1 Structure of HCB
Table : 3 Physical and chemical properties of HCB.
Property Value
Molecular mass 284.78
Melting point oC 231
Boiling point oC 325
Vapour pressure mm Hg at 20 oC 1.09 x 10-5 Water solubility mg /litre at 20 oC 0.005815 Log octanol/water partition coefficient 5.73
Density at 23oC g/cm3 2.044
Flash point oC 242
Cl
Cl
Cl Cl
Cl
Cl
2.3.2. Sources in the environment
HCB was used in the past as fungicide, wood preservative, intermediates in organic syntheses, synthetic rubber peptizing agent and so on [89]. HCB was first introduced in 1945 for its agricultural use as a fungicide [90]. The peak HCB production was during the late 1970s and early 1980s when the annual production was about 10,000 tonnes per year from 1978 to 1981 [90]. However, it was recognized as a hazardous chemical following an episode of massive human poisoning in Turkey 1959 which resulted from the consumption of bread prepared from wheat contaminated with HCB. The global production of HCB marked its decline due to several restrictions on its use and from 1970s several countries declared a ban on its production. At present, there is a global ban existing on its manufacture. However, the release of HCB still continues as it is produced as a byproduct during the manufacture of several chlorinated hydrocarbons as well as other sources. An estimate given by Bailey in 2000 indicates that a total of about 23000 kg per year of new HCB added to the environment which comes mainly from trace contamination of pesticides, combustion, manufacturing and biomass burning. However, this sum would be exclusive of the sources of HCB still existing in developing countries [87].
HCB is widely dispersed in the environment. Though significant quantities of HCB are present in the atmosphere, the surface oceans and the soil, the major environmental reservoir of HCB was identified as the soil [90, 91]. This is because of the hydrophobic nature of HCB which allows itself to partition into soil and plant surfaces. Therefore, soil serve as a sink by receiving these pollutants through numerous pathways including direct pesticide application, atmospheric deposition, application of sewage sludge or compost, spills, erosion from nearby
contaminated areas and contaminated water irrigation [90]. Some examples of HCB contaminated area are an industrial area at Bitterfeld, Germany and Lake Päijänne, Finland [92, 93].
From all these, it is evident that HCB is very persistent in the environment and draws concern for its removal from the environment, especially soil.
2.3.2 Health effects
Some of the harmful health effects associated with chronic exposure to HCB are porphyria, thyroid imbalances and cancer [94]. It is directly linked to reproductive, developmental, behaviourial, neurological, and endocrine mal functions [95, 96]. TDI values for HCB have been suggested by various governmental agencies such as the USEPA and ATSDR and vary in the range 1.6E -04 mg/kg d to 5E-05 mg/kg d [97]. There have been several studies on the association between liver, immunological and renal effects and human exposure to HCB by inhalation and oral ingestion. However, no data could be found regarding the health effects in humans and animals upon dermal exposure to HCB [98]
3. OBJECTIVES OF THE STUDY
The objective of this research was to investigate and further develop a method for the removal of sorbed organic contaminants from low permeable soil. For that, a stepwise and methodological approach was adopted to carry out a series of experiments for evaluating the process performance and contaminant removal efficiency during the electrokinetic Fenton process. Kaolin was used as the model low permeable soil and the representative hydrophobic organic compound chosen for the study was HCB.
The scope of this research included;
I Investigating the feasibility of electrokinetic and electrokinetic Fenton processes for the removal of HCB from soil (Paper I).
II Studying the influence of electrode positions in electrokinetic Fenton system (Paper II).
III Studying the effect of cyclodextrin as an enhancing agent (Paper III).
IV Studying the soil-oxidant interaction at different conditions (Paper IV).
V Studying the effect of electrode polarity reversal on the contaminant removal (Paper V).
4. MATERIALS AND METHODS
4.1 Design of apparatus
The apparatus designed for carrying out the experiments had to meet the following requirements:
1. Allow the handling and processing of the test specimen without actually disturbing the whole system.
2. Resemble a practical field unit rather than a laboratory set-up 3. Enable the scale-up of the actual set-up
4. Provide a safe working condition
Three different designs were used for the present study. The designs, basically of same structural type differed only by certain features which were incorporated to suit the experimental conditions as the studies proceeded (Fig: 2).
Fig: 2 Basic structure of the apparatus designed for the experiments
Type 1 apparatus consisted of three parts: two electrode chambers filled with electrolyte or oxidant solutions and a soil chamber in between. Type 2 apparatus consisted of a single soil
power supply cathode
effluent
anode effluent
cathode anode
Soil Chamber
chamber in which the electrodes were immersed. Oxidants and DW were directly applied on the soil surface. Type 3 apparatus was similar to Type 1, but with injection wells incorporated to them. Injection wells (2 cm diameter) were made of perforated polyvinyl chloride tubes and covered with nylon cloth and then inserted to the soil chamber.
Type 1 and 3 apparatus were made of glass or acrylic and type 2 of HDPE. The electrode chambers were separated from soil chamber using filter cloth or nylon cloth. This allowed the permeation of fluid while preventing soil to pass through them. The chambers were closed using a removable lid with openings to insert electrodes which also served as gas vents. Inert electrodes made of titanium and coated with platinum were used for all experiments. The electrodes were connected to a DC power supply providing a constant voltage of 1.5 or 2.0 V/cm.
4.2 Soil characterisation
Commercially obtained kaolin was used as the model soil for the experiments. The physical and chemical characterization of kaolin was performed in the laboratory following the methods described by Rowell [99] and is presented in Table 4.
Table 4: Kaolin characteristics
Properties Values
Mineralogy
Kaolin 100 % Paricle size distribution
Gravel % 0 Sand % 7 Silt % 17 Clay % 76
Specific Gravity 0.508
Carbonate content % 5.5
pH 5.2
Electrical conductivity ( µS) 189.2 Cation exchange capacity (m mol/100g) 3
4.3 Test specimen
Contaminated kaolin used as the test specimen was prepared by artificially spiking the kaolin with HCB. For this, HCB/hexane solution was prepared. It was then added to dry kaolin so as to get a target concentration of 100 mg/kg, mixed well and left in the fume hood till all the hexane had evaporated and the soil was dry. The dry contaminated soil was thoroughly mixed to get a homogeneous mixture and then brought to the required moisture content of approximately 40 % by adding DW. The test specimen was then stacked into the soil chamber and samples were taken from different points to determine the initial pH and HCB concentration.
4.4 Experimental Program
The experimental program is summarized as a schematic diagram and shown in Fig. 3. The experiments were divided into five different phases with specific objectives as indicated in the figure. Experiments were done both with and without added Fe. Fe when added was supplied as ferrous sulphate solution prepared with a Fe to substrate mass ratio of 1:10. The concentration of beta-cyclodextrin when used was 1 % (wt %), which is well above its critical micelle concentration. The anolyte in the experiments were either deionized water or the oxidant itself.
The catholyte in all experiments was DW. The electrodes were connected to a power supply in each case after assembling the apparatus and setting up the test. A constant applied voltage of 1.5 V/cm or 2 V/cm was used. H2O2 was used in different concentrations ranging from 5 to 30 %. All the chemicals were freshly prepared and the dilutions were made with DW.
Fig: 3 Schematic representation of the experimental program (Paper I)
Kaolin Characterisation
Third Phase Experiments
Electrokinetic Fenton Treatment with Cyclodextrin
Using 5, 15 and 30 % H2O2
Electrokinetic Fenton Treatment without
Cyclodextrin
Using 5, 15 and 30 % H2O2
Fifth Phase Experiments
Electrokinetic Fenton Treatment with polarity reversal using added and inherent Fe
Oxidant addition from anode
Oxidant addition from multiple points
Fourth Phase Experiments
Oxidant addition from multiple points
Electrokinetic Fenton Treatment with different modes and point of oxidant addition using added and inherent Fe
Oxidant addition from anode Second Phase
Experiments Electrode Positioning
Type 1 Apparatus
Type 2 Apparatus First Phase
Experiment s
Electrokinetic Treatment
Electrokinetic Fenton Treatment Feasibility Studies
Sorption studies
(Paper II)
(Paper III)
(Paper IV)
(Paper V)
Sorption studies – A series of batch adsorption tests were performed to determine the sorption capacity of HCB to kaolin.
First Phase - Three tests were carried out during the first phase, of which two were electrokinetic experiments to observe the suitability of β-cyclodextrin as a flushing solution for these processes. The third experiment was electrokinetic Fenton test using β-cyclodextrin as an enhancing agent (Paper I).
Second Phase - The influence of electrode positions in the system was compared by selecting two different kinds of apparatus. In Type 1 apparatus, the electrodes were contained in separate chambers. In Type 2 apparatus, the electrodes were directly immersed into the soil mass. β-cyclodextrin was used in both cases (Paper II).
Third Phase - Experiments were conducted in two series for fifteen consecutive days: First series experiments consisted of three tests with different concentrations of H2O2 in the absence of cyclodextrin. Test 1 with 30 % H2O2, Test 2 with 15 % H2O2, and Test 3 with 5 % H2O2. Second series tests also consisted of three tests with the same different concentrations of H2O2, but in the presence of cyclodextrin. All the experiments were carried out in Type 2 apparatus without adding any Fe to the system. Inherent iron in Kaolin was expected to catalyze the H2O2 during the Fenton’s oxidation (Paper III).
Fourth phase Experiments - A series of electrokinetic Fenton experiments have been carried out to investigate the effect of oxidant delivery and availability on contaminant removal and its subsequent impact on the treatment duration (Paper IV).The operating conditions of the experiments are shown in the Table: 5
Table: 5 Operating conditions of fourth phase experiments
Fifth Phase - A series of experiments were carried out to explore the possibilities of using polarity reversal as an enhancement during electrokinetic Fenton treatment of HCB contaminated soil (Paper V).
Details of the experiments are shown in the Table: 6 Experi
ment Voltage
(V/cm) Anolyte Injection
well Catholyte Mode of oxidant delivery Duration (Days)
1 1.5 H2O2 Nil DW from anode 20
2 1.5 H2O2
+FeSO4
Nil DW Serial addition one after the other from the anode 20 3 1.5 H2O2 +
FeSO4
Nil DW FeSO4 added as the anolyte and then added H2O2 after five daysto the anode
15
4 1.5 H2O2
+FeSO4
Two wells at distances 4-6 cm from both the walls
DW FeSO4 added as the anolyte and then added H2O2 after five daysto the anode and wells
15
5 2 H2O2
+FeSO4
One well at the center of the soil mass
DW FeSO4 added as the anolyte and then added H2O2 after two daysto the anode and well.
10
Table: 6 Operating conditions of fifth phase experiments
4.5 Chemical analyses
pH and redox measurements
The pH and redox of the soil samples were measured using a pH meter and a redox meter (WTW 340i equipped with SenTix 61 and SenTix ORP sensors, pH 730 inoLab WTW series). The soil sample for the pH and redox measurements were made into a suspension of the sample in DW in the ratio 1 : 25
Exper
iment Voltage
(V/cm) Anolyte Catholyte Mode of oxidant addition
Injection well Duration
(days) Polarity reversal
1 1.5 H2O2 DW H2O2 added
from the anode nil 10 After 5
days
2 1.5 DW DW H2O2 added to
the injection well
One in center, cylindrical, 2cm diameter
10 After 5
days
3 1.5 H2O2 DW H2O2 added
from the anode nil 10 nil
4 1.5 FeSO4,
H2O2
DW FeSO4 added as the anolyte and then added H2O2 after two daysto the anode
nil 10 After 5
days
5 1.5 FeSO4,
H2O2
DW FeSO4 added as the anolyte and then added H2O2 after two days to the anode and injection well
One in center, cylindrical, 2cm diameter.
20 After 10
days
Electroosmotic flow
The electroosmotic flow was measured manually by measuring and balancing the additions made to the analyte and the volume removed from the catholyte. In few cases, the zeta potential of the soil samples was measured and the electroosmotic flow was calculated based on that. The zeta potential of the soil samples were measured by Zeta sizer Nano series (Malvern instrument), equipped with a microprocessor unit. A strong correlation existed between the values calculated using the zeta potential values and those measured manually.
HCB Analysis
HCB was extracted from the soil sample by ultrasonication based on a method adopted from Yuan et al. [35]. The extract so obtained was analyzed using a gas chromatograph coupled to an inert mass selective detector (Agilent 5975). The column used was HP-5 capillary column (30 x 0.32 mm ID) with a 0.25 μm film thickness. Helium at constant flow rate (25 cm /s) was used as carrier gas. The oven temperature was programmed from 40oC to 270oC at 10 oC/min. The injector temperature used was 250oC and the injection volume was 1 μL. Quantification was based on a linear curve made with four or five standard solutions of HCB. All the extractions and sample runs were done in duplicate to ensure the reliability of the measurements.
Other
Apart from the above analyses, the H2O2concentrations in the soil samples were analyzed in some experiments using the permanganate method. The data obtained were used to ensure the presence of H2O2 in the soil sample and not directly used to interpret the results.
5. RESULTS AND DISCUSSION
5.1 Sorption studies
The sorption data fitted to the following equation and seemed to follow a linear equation:
Q = 1.3599C (7)
This is in good agreement with the results obtained by Schwarzenbach et al. [100] and Means et al. [101] who stated that the sorption of nonpolar organic compounds of low to intermediate lipophilicity by aquifer materials and the sorption of other PAHs on different sediment and soil substrates followed linear equilibrium isotherms.
5.2 Feasibility Tests
The preliminary feasibility tests included the experiments carried out to evaluate the suitability of electrokinetic and electrokinetic Fenton processes to treat HCB contaminated soil (Paper I). The results of the experiments showed that β-cyclodextrin could solubilise the sorbed HCB and transport them through the soil matrix. This result is in agreement with Yuan et al. [35] who demonstrated that HCB can be desorbed and mobilized by β-cyclodextrin. The electrokinetic Fenton test also resulted in an overall average removal of 64% HCB from the soil.
5.3 Electrode positioning
The influence of electrode positions in the system was compared by selecting two different kinds of apparatus, Type 1 and Type 2 (Paper II). β-cyclodextrin was used in both cases and hence, as the experiment proceeded, the sorbed HCB would have desorbed and the oxidation of HCB had occurred both in the sorbed and desorbed state. The electrode positions seemed to have drastically changed the soil pH. The soil pH in Type 1 apparatus near anode and cathode was
about 3 at the end of the experiment. In Type 2 apparatus, the soil pH near the anode dropped to 3 towards the end of the experiment and near the cathode the pH rose gradually and was 9.6 at the end of the experiment. This difference in the pH distribution had a significant effect on the contaminant removal (Fig. 4).
Fig: 4 Rate of contaminant removal. (Data from Paper II)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
1 4 8 12
cumulative removal
Elapsed Time (days)
Type 1 Type 2
Fig: 5 HCB distribution along the anode and cathode region in Type 1 apparatus. (Data from Paper II)
Fig: 6 HCB distribution along the anode and cathode region in Type 2 apparatus. (Data from Paper II)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
1 4 8 12
C/C0
Elapsed Time (Days) Type 2 - Anode Type 2 - cathode
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
1 4 8 10 14
C/C0
Elapsed Time (Days) Type 1 - Anode Type 2 - Cathode
